Processing images of at least one living being

ABSTRACT

A method of processing images of at least one living being, includes obtaining a sequence ( 19 ) of digital images taken at consecutive points in time. At least one measurement zone ( 26 ) comprising a plurality of image points is selected. For each measurement zone ( 26 ), a signal ( 28,30 ) representative of at least variations in a time-varying value of a combination of pixel values at least a number of the image points for use in determining at least one of a presence and a frequency value of at least one peak in a spectrum of the signal ( 28,30 ) corresponding to a frequency of a periodic physiological phenomenon is obtained. The step ( 25 ) of selecting at least one measurement zone ( 26 ) includes analyzing information based on pixel data of a plurality of image parts in at least one of the images ( 19 ), each image part including at least one image point, and selecting each measurement zone ( 26 ) from contiguous parts determined to have similar characteristics.

FIELD OF THE INVENTION

The invention relates to a method of processing images of at least oneliving being, including:

obtaining a sequence of digital images taken at consecutive points intime;

selecting at least one measurement zone comprising a plurality of imagepoints; and

for each measurement zone, obtaining a signal representative of at leastvariations in a time-varying value of a combination of pixel values atleast a number of the image points for use in determining at least oneof a presence and a frequency value of at least one peak in a spectrumof the signal corresponding to a frequency of a periodic physiologicalphenomenon.

The invention also relates to a system for processing images of at leastone living being, including:

an interface for obtaining data representative of a sequence of digitalimages taken at consecutive points in time; and

an image data processing system, configured to:

select at least one measurement zone comprising a plurality of imagepoints; and

for each measurement zone, to obtain a signal representative of at leastvariations in a time-varying value of a combination of pixel values atleast a number of the image points for use in determining at least oneof a presence and a frequency value of at least one peak in a spectrumof the signal corresponding to a frequency of a periodic physiologicalphenomenon.

The invention also relates to a computer program.

BACKGROUND OF THE INVENTION

Verkruysse et al., “Remote plethysmographic imaging using ambientlight”, Optics Express, 16 (26), 22 Dec. 2008, pp. 21434-21445demonstrates that photo-plethysmography signals can be measure remotelyon the human face with normal ambient light as the source and a simpledigital, consumer-level photo camera in movie mode. After setting thecamera in movie mode, volunteers were asked to sit, stand or lie down tominimize any movements. Color movies were saved by the camera andtransferred to a personal computer. Pixel values for the red, green andblue channels were read for each movie frame, providing a set ofPV(x,y,t), where x and y are horizontal and vertical positions,respectively and t is time corresponding to the frame rate. Using agraphic user interface, regions of interest (ROI) were selected in astill (selected from the movie) and the raw signal PV_(raw)(t) wascalculated as the average of all pixel values in the ROI. Fast FourierTransforms were performed to determine the power and phase spectra. Itis stated that selection of the ROI is not critical for the heart ratedetermination. It is also stated that limits to the spatial resolutionof photoplethysmography images due to movement artifacts may be solvableby improved positioning of the volunteers, software to laterallysynchronize the frames and more homogeneous illumination to reduceshading artifacts.

A problem of the known method is that it uses a supervisor to select theROI of the part of the image he knows corresponds to the living person.

It is an object of the invention to provide a method, system andcomputer program of the types mentioned above in the opening paragraphsthat require little or no human supervision in order to provide goodresults.

This object is achieved by the method according to the invention, whichincludes:

obtaining a sequence of digital images taken at consecutive points intime;

selecting at least one measurement zone comprising a plurality of imagepoints; and

for each measurement zone, obtaining a signal representative of at leastvariations in a time-varying value of a combination of pixel values atleast a number of the image points for use in determining at least oneof a presence and a frequency value of at least one peak in a spectrumof the signal corresponding to a frequency of a periodic physiologicalphenomenon. The step of selecting at least one measurement zone includesanalyzing information based on pixel data of a plurality of image partsin at least one of the images, each image part including at least oneimage point, and selecting each measurement zone from contiguous partsdetermined to have similar characteristics.

Analyzing information based on pixel data of a plurality of image partsin at least one of the images, where each image part includes at leastone image point, can be conducted automatically, as can clustering thoseparts determined to have similar characteristics. Thus, this method issuitable for unsupervised execution. Selecting contiguous partsdetermined to have similar characteristics results in the determinationof a region of the image with homogeneous characteristics. If thesecharacteristics are similar according to an analysis in the spatialdomain, a better selection of a homogeneous zone which will form themeasurement zone can be made. Even if the body part corresponding to themeasurement zone does not remain exactly in position throughout thesequence of images, the pixel intensities in the measurement zone willnot vary appreciably due to such variations in position. This improvesthe quality of the spectrum of signal corresponding to the time-varyingvalue of the combination of pixel values at least a number of the imagepoints, so that reliable identifications of signal peaks correspondingto heart beat or breathing rate can be made. The effect is not dependenton particular lighting conditions, making the method more robust andmore suitable for remote sensing applications. By using datarepresentative of at least part of a spectrum of a time-varying value ofa combination of pixel values at least a number of the image points, alarge amount of noise can be eliminated. This allows one to use imagesthat are obtained by capturing light reflected off a living subject.Such images can be obtained with a relatively cheap camera or sensorarray. By contrast, if one were to determine the spectrum of each pixelindividually and then cluster the values of the peaks, one would have touse images obtained using a very sensitive imaging device, e.g. apassive thermal imaging device.

An embodiment of the method includes performing image segmentation on atleast one of the sequence of digital images to select pixel data for theanalysis included in the selection step.

An effect is that the amount of pixel data that has to be analyzed inthe selection step is reduced, since only certain promising ones of thesegments obtained in the segmentation step need be processed.

In a variant of this embodiment, the image segmentation is performedusing an algorithm for recognizing image parts corresponding to at leastone type of body part of a living being.

This variant selects those image parts corresponding to parts of livingbeings that are suitable for an analysis to determine at least one of apresence and a frequency value of at least one peak in a spectrum of theaverage brightness signal corresponding to the frequency of a periodicphysiological phenomenon. In principle, the method is based on the factthat the intensity of light reflected off skin varies with the frequencyof periodic physiological phenomenon, i.e. the heart rate andrespiration rate. Thus, a segmentation algorithm aimed at detectingskin, or body parts with shapes corresponding to those of body partsthat are generally uncovered (e.g. the face of a human being) provides apre-selection of suitable image segments, within which one or morehomogeneous measurement zones are selected.

An embodiment of the method includes using a tracking algorithm to placeat least one of the measurement zone and an image segment including themeasurement zone in each of a plurality of the images in the sequence.

This embodiment takes account of the fact that even a homogeneousmeasurement zone may be affected by larger movements. The trackingalgorithm allows the measurement zone to move with the actual body partit represents. Thus, signal artifacts arising from inhomogeneous imageparts moving into the measurement zone are largely avoided. Thisimproves the signal to noise ratio of the signal componentscorresponding to the periodic physiological phenomena.

In an embodiment, the sequence of digital images is caused to becaptured by a camera upon completion of an initialization phase, theinitialization phase including:

measuring periodic intensity fluctuations in at least parts of imagesacquired by the camera whilst camera settings are varied, and

selecting values of the camera settings at which measured periodicintensity fluctuations in at least a range of frequencies are determinedto be minimal.

This embodiment allows one to remove sources of periodic disturbances,e.g. at the mains frequency. Typically, such disturbances correspond toperiodic fluctuations in ambient lighting. Because the method issuitable for remote imaging, these disturbances play more of a role thanwould be the case if one were to use e.g. an infrared light source andcamera. Intensity fluctuation measurements can be limited to one colorcomponent or be based on a weighted sum of some or all of the colorcomponents comprised in the pixel data. Suitable camera settings to bevaried include the frame rate, exposure time, camera gain and pixelclock.

An embodiment of the method includes determining a correction signalcorresponding to a time-varying value of a combination of pixel valuesat least a number of image points in an image part other than themeasurement zone, and

decorrelating at least one of the pixel data of the images in at leastthe measurement zone and the time-varying value of the combination ofpixel values at least a number of the image points in the measurementzone from the correction signal.

This embodiment allows one to move non-periodic disturbances from theanalysis, further improving the signal to noise ratio of the signalcomponents due to periodic physiological phenomena. An example would bethe reflections of a television signal in the face of a person watchingtelevision whilst the sequence of images is captured. It is noted thatthe image part other than the measurement zone may be a larger imagepart that also encompasses the measurement zone.

In an embodiment of the method, an output representative of whether thepresence of at least one peak in the spectrum corresponding to afrequency of the periodic physiological phenomenon is detected is usedto control a device arranged to perform a function conditional ondetecting a presence of at least one living being of at least one kind.

An effect is that it is possible to verify that a living person isactually present in an unobtrusive way.

In a variant, the output is provided to a conditional access system foruse in an authentication operation.

An effect is that it is no longer possible to fool the conditionalaccess system that a particular person is present. For example, it isnot possible to provide a fingerprint detector with a wax cast of anabsent person's finger, or to fool a face recognition system with aphotograph of an absent person. Thus, this method is particularlysuitable for use in conjunction with a biometric conditional accesssystem.

In an embodiment of the method, the frequency value of at least one peakin the spectrum corresponding to the frequency of the periodicphysiological phenomenon is determined and a system for providingperceptible output is caused to adapt its output in dependence on thefrequency signal.

Thus, this embodiment is particular suited to providing bio-feedback,e.g. in an ambient system or in a gaming or fitness environment.

An embodiment of the method includes providing a gating signal based onthe signal corresponding to at least the variation in the time-varyingvalue of the combination of pixel values at least a number of the imagepoints to an imaging system.

This embodiment is suitable for use with imaging systems such as MRI orCT systems, wherein a person is placed in a scanner. In such imagingsystems, a gating signal is often required in order to obtain a stillimage of e.g. a heart. An unobtrusive heart or respiration ratedetermination is preferred to lower the stress level of the person beingimaged. Furthermore, there are no wires or probes that might affect theimaging system.

An embodiment of the method includes:

selecting a plurality of measurement zones;

for each measurement zone, obtaining a signal representative of at leastvariations in a time-varying value of the combination of pixel values atleast a number of the image points and determining the frequency valueof at least one peak in a spectrum of the signal corresponding to afrequency of a periodic physiological phenomenon; and

detecting how many living beings are represented in the sequence ofimages by comparing the frequency values.

This method is suitable for an image segmentation method for example. Itallows one to discern between different people in the images. Otherapplications include image analysis systems for crowd control, forexample.

According to another aspect, the system for processing images of atleast one living being according to the invention includes:

an interface for obtaining data representative of a sequence of digitalimages taken at consecutive points in time; and

an image data processing system, configured to:

select at least one measurement zone comprising a plurality of imagepoints; and

for each measurement zone, to obtain a signal representative of at leastvariations in a time-varying value of the combination of pixel values atleast a number of the image points for use in determining at least oneof a presence and a frequency value of at least one peak in a spectrumof the signal corresponding to a frequency of a periodic physiologicalphenomenon, wherein the image data processing system is configured toselect the at least one measurement zone by analyzing information basedon pixel data of a plurality of image parts in at least one of theimages, each image part including at least one image point, and toselect each measurement zone from contiguous parts determined to havesimilar characteristics.

In an embodiment, the system is configured to carry out a methodaccording to the invention.

According to another aspect of the invention, there is provided acomputer programme including a set of instructions capable, whenincorporated in a machine-readable medium, of causing a system havinginformation processing capabilities to perform a method according to theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in further detail with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic diagram of a system arranged to adapt its outputin dependence on whether it has detected the presence of a living beingor on the heart rate or respiration rate of a living being;

FIG. 2 is a flow chart illustrating a method for determining the heartrate or respiration rate of the living being; and

FIG. 3 is a schematic diagram of an imaging system that is gated by asignal obtained by processing a running sequence of images.

DETAILED DESCRIPTION

Referring to FIG. 1, an example is given here of a first system 1 thatis arranged to process a sequence of images. The first system 1 carriesout this processing in order to determine at least one of the presenceand a frequency value of at least one peak in a spectrum of a signalbased on the pixel data of the images corresponding to a frequency of aperiodic physiological phenomenon. The presence of a living being isinferred from the presence of the peak, and used as a binary input forone or more system processes. The frequency of the peak is used as aninput in at least one of those processes.

The first system 1 includes a digital camera 2 arranged to record asequence of digital images in quick succession. The first system 1further includes a data processing device 3 which is in communicationwith the digital camera 2 for the purpose of obtaining the image data,but also to control the operation of the digital camera 2, as will beexplained.

The digital image data captured by the digital camera 2 is passed to thedata processing device 3 via an interface 4 of the data processingdevice 3. In the illustrated embodiment, the data processing device 3includes a processing unit 5 and main memory 6, as well as a datastorage device 7 for non-volatile storage of data, e.g. the digitalimage data and software enabling the data processing device 3 to processthe image data and control a number of peripheral devices.

In the illustrated embodiment, the peripheral devices include atelevision set 8; an ambient system including a controller 9 andlighting units 10-12; and a biometric scanning device 13. All areconnected to the data processing device 3 via respective interfaces14-16. These peripheral devices are just examples of peripheral devicesthat can be controlled in dependence on the results of one or morevariants of an image processing method to be described with reference toFIG. 2.

This method is used to determine the presence of a living being, i.e. ahuman or animal, in a scene captured by the digital camera 2, bygenerating a signal on the basis of image data corresponding to a patchof skin, which signal varies with the frequency of a periodicphysiological phenomenon, e.g. the heartbeat or breathing of a humanbeing.

The human skin can be modeled as a two-layered object, one layer beingthe epidermis (a thin surface layer) and the other the dermis (a thickerlayer underneath the epidermis). Approximately 5% of an incoming ray oflight is reflected in the epidermis, which is the case for allwavelengths and skin colors. The remaining light is scattered andabsorbed within the two skin layers in a phenomenon known as bodyreflectance (described in the Dichromatic Reflection Model). Theepidermis behaves like an optical filter, mainly absorbing light. In thedermis, light is both scattered and absorbed. The absorption isdependent on the blood composition, so that the absorption is sensitiveto blood flow variations. The optical properties of the dermis aregenerally the same for all human races. The dermis contains a densenetwork of blood vessels, about 10% of an adult's total vessel network.These vessels contract according to the blood flow in the body. Theyconsequently change the structure of the dermis, which influences thereflectance of the skin layers. Consequently, the heart rate can bedetermined from skin reflectance variations, which is the principleunderlying the method presented herein.

In the illustrated embodiment of the method, an initialization phase iscompleted first, in order to determine the appropriate settings for thedigital camera 2 (step 17). To this end, the data processing device 3causes at least one of the frame rate, exposure time, pixel clock(determines the rate at which pixel values are acquired) and gain of thecamera channel of the digital camera 2 to be varied whilst a sequence ofdigital images is captured. The (spatial) average brightness of at leastpart of each image of the sequence is determined, and the magnitude ofthe periodic fluctuations in the average brightness is determined foreach new value of the settings. Those settings for which the magnitudewithin at least a range of the spectrum, in particular a low-frequencyrange, is smallest are selected for subsequent use in the method.Instead of determining the spatial average brightness of at least a partof the image, an individual pixel's brightness fluctuations can bedetermined. The effect of choosing the settings of the digital camera 2is that periodic background lighting fluctuations are absent to thelargest extent possible from the sequence of images to which theremainder of the method is applied.

In a next step 18, a sequence 19 of images is obtained from the digitalcamera 2. The sequence 19 of images represents a scene captured atsuccessive points in time, which may be at regular or irregularintervals.

In a next step 20, the images 19 are processed in order to removenon-periodic background signals. To this end, a correction signalcorresponding to a time-varying average brightness of part or all of theimages 19 is formed. In the illustrated embodiment, the pixel data ofthe images 19 is then decorrelated from the correction signal.Algorithms for cancelling non-linear cross-correlations are known perse. Further image processing may take place at this stage 20, e.g. tocompensate for camera motion.

In two next steps 21,22, an image segmentation algorithm is performed onat least one image of the sequence 19 of digital images. In particular,an algorithm for detecting an image segment 23 representing a body part,generally the face, is carried out in these steps 21,22. A suitablealgorithm is described in Viola, P. and Jones, M. J., “Robust real-timeobject detection”, Proc. of IEEE Workshop on statistical andcomputational theories of vision, 13 Jul. 2001. Other suitablealgorithms based on recognizing segments with certain shapes and/orcolors (skin colors) are known and can be used instead of or in additionto this algorithm. One or more, for example all, distinct segments 23determined to correspond to a body part of the desired type are tracked(step 24) through the sequence 19 of images. That is to say that thesegment 23 is placed, i.e. its location determined, by comparing theimages in the sequence 19 to quantify the movement of the body partswithin the images 19. A suitable tracking algorithm is known, forexample, from De Haan et al., “True-motion estimation with 3-D recursivesearch block matching”, IEEE Transactions on circuits and systems forvideo technology, 3 (5), October 1993, pp. 368-379.

Subsequently, for each selected and tracked segment 23 a measurementzone 26 within the image segment 23 is selected (step 25). This step 25involves a spatial analysis of the pixel data of a plurality of imageparts—each image part being one or more image points in size—todetermine a set of contiguous parts determined to have similarcharacteristics. This step 25 is carried out automatically by the dataprocessing device 3. A suitable algorithm is an algorithm for detectingregions with minimal gradient variations. Those image parts belonging tothe region are selected to form the measurement zone 26. In theillustrated embodiment, the position of the measurement zone 26 isdetermined by analysis of a key image in the sequence 19 of images. Itsposition relative to the segment 23 corresponding to a body part isdetermined, and it is thus tracked with the image segment 23 through thesequence 19 of images. Thus, it is determined which pixel of each of theimages corresponds to a particular image point of the measurement zone26 for all image points making up the measurement zone.

Next (step 27), a signal 28 representative of the time-varying averagebrightness of the pixels corresponding to the image points of themeasurement zone 26 is generated. For each image of the sequence 19, theaverage brightness of the pixels determined to be comprised in themeasurement zone 26 is formed. Since each image of the sequence 19represents a point in time, a time-varying (discrete-time) signal isthus obtained. In an alternative embodiment, certain image points arediscarded, so that a sum of pixel values at fewer than all image pointsin the measurement zone 26 is taken. Moreover, the brightness may be aweighted sum of the color components or only the value of one colorcomponent. Green has been found to have the strongest signal.

The signal 28 is then centered on its mean value (step 29) to yield afurther signal 30 representative of the time-varying average brightnessof pixels corresponding to the image points of the measurement zone 26,the better to observe variations in it. In a variant, this step 29 alsocomprises the decorrelation with the correction signal that isalternatively comprised in the step 20. In a different variant, thisstep 29 comprises a filtering operation, e.g. a filtering operationcorresponding to differentiation of the signal. Other alternatives forextracting variations of the order of 1% of the first signal's dynamicrange are possible.

Finally (step 31) basic signal processing techniques are used to extractinformation representative of the heart rate or respiration rate fromthe second signal 30.

A first application of at least part of the method of FIG. 2 in thesystem of FIG. 1 involves detecting the presence of a living being, inparticular a human being. To this end, an output representative ofwhether the presence of at least one peak in the spectrum correspondingto a frequency of the periodic physiological phenomenon is detected isused to control one or more of the peripheral devices to perform afunction conditional on detecting a presence of at least one humanbeing. In this case, the spectrum, or peaks of the spectrum, in at leasta limited range is compared with a pre-determined reference rangecorresponding to typical human heart rates or respiration rates. If ahuman being is present, then, for example, the television set 8 and theambient system can continue to function. If not, they can be switchedoff or switched to a standby function. Thus, this application is anapplication in an energy-saving device. A similar application is tointelligent lighting systems for homes and offices. The detection ofliving beings by means of automated image analysis is less sensitive tofalse alarms, e.g. due to pets.

A similar application is to control a conditional access system, e.g.one including the biometric scanning device 13. In an embodiment, thiscan be a fingerprint scanner. Using the detection of living beings, itis ensured that e.g. wax casts of a person's finger cannot be used tofool the conditional access system. A conditional access system usingonly the camera 2 (e.g. to scan a person's iris or face) can alsobenefit from the additional use of the method of FIG. 2 to determinethat a living person is actually in front of the camera lens.

Alternatively or additionally, the method of FIG. 2 is used to providebiofeedback to a user. More particularly, at least the ambient system iscaused to adapt its output in dependence on the frequency determined inthe last step 31 of the method. For example, the color or intensity oflight emitted by the lighting units 10,11,12 can be changed independence on the heart rate. To this end, the method of FIG. 2 iscarried out in real-time on a sequence comprising the last N digitalimages obtained by the data processing device 3. N depends on the imagecapture rate, and is chosen in dependence on the image capture rate tocover a sequence spanning a time interval long enough to cover at leasttwo heartbeats of an average human being, e.g. at least four secondslong. In a variant, multiple measurement zones 26 are selected, andmultiple average signals 30 are established, so that the data processingdevice 3 is able to determine the current heart rate and/or respirationrate of multiple individuals. Thus, the feedback provided using theambient system can be made dependent on the heart rate of multipleusers.

An alternative application of at least a part of the method of FIG. 2involves the use of a system 33 as illustrated schematically in FIG. 3.In this application, the average signal 28 or the signal 30representative of the time-varying average brightness but centered onthe mean value thereof is used as a gating signal for an imaging system.The imaging system can be an MRI (Magnetic Resonance Imaging) or CT(Computer Tomography) scanner system, for example. Such an imagingsystem captures multiple two-dimensional cross-sectional views of apatient. In order to correct for motion of the patient or the patient'sorgans, the image capturing process is gated using a signalrepresentative of the periodic physiological phenomenon that causes theperiodic movement. In the illustrated system 33, which executes at leastthe first nine steps 17,18,20-22,24,25,27,29 of the method of FIG. 2,the signal 30 that corresponds to the variations in the averagebrightness of the automatically selected measurement zone is used togate an image capturing device 32 and a pulse transmitter 34, which arecontrolled by a data processing device 35 by means of signals providedthrough appropriate interfaces 36,37. The data processing device 35comprises a data processing unit 38, main memory 39 and data storageunit 40. It receives the digital image data recording ambient lightreflected off the patient from a digital camera 41 through a furtherinterface 42. Images formed by processing of the data from the imagecapturing device 32 are displayed on a monitor 43, using an appropriategraphics controller 44. With the method of FIG. 2 (excluding the laststep 31), the gating signal for the medical imaging method is obtainedin an unobtrusive way. First, the experience for the patient is lessstressful, because there are no sensors attached to the patient. Second,wires or sensors cannot affect the operation of the transmitter 34 orimage capturing device 32. The process by which the gating signal isacquired is completely automated, so that the attention of the medicalpersonnel is not taken up with selection of the measurement zone 26, andthe monitor 43 is used only for the actual medical imaging method. Theuse of a measurement zone 26 and the averaging over it (step 31) ensuresthat the gating signal is relatively free from noise. It thus requireslittle or no filtering.

It should be noted that the above-mentioned embodiments illustrate,rather than limit, the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The word “comprising” does not exclude the presence of elements or stepsother than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.The mere fact that certain measures are recited in mutually differentdependent claims does not indicate that a combination of these measurescannot be used to advantage.

As mentioned, the method outlined herein can be carried out on a singlecolor component of the pixel data or on a weighted sum of two or more ofthe color components (Red, Green and Blue or Cyan, Magenta, Yellow andKey, for example).

The medical imaging method described with reference to FIG. 3 can alsoinvolve the execution of the last step 31 of the method shown in FIG. 2,in which case it is used to control the frequency of a gating signalprovided by a signal generator.

1. Method of processing images of at least one living being, including:obtaining a sequence (19) of digital images taken at consecutive pointsin time; selecting at least one measurement zone (26) comprising aplurality of image points, wherein the step (25) of selecting at leastone measurement zone (26) includes analyzing information based on pixeldata of a plurality of image parts in at least one of the images (19),each image part including at least one image point, and selecting eachmeasurement zone (26) from contiguous parts determined to have similarcharacteristics; and for each measurement zone (26), obtaining a signal(28,30) representative of at least variations in a time-varying averagevalue of a combination of pixel values at least a number of the imagepoints for use in determining at least one of a presence and a frequencyvalue of at least one peak in a spectrum of the signal (28,30)corresponding to a frequency of a periodic physiological phenomenon. 2.Method according to claim 1, including performing image segmentation onat least one of the sequence (19) of digital images to select pixel datafor the analysis included in the selection step (25).
 3. Methodaccording to claim 2, wherein the image segmentation is performed usingan algorithm for recognizing image parts (23) corresponding to at leastone type of body part of a living being.
 4. Method according to claim 1,including using a tracking algorithm to place at least one of themeasurement zone (26) and an image segment (23) including themeasurement zone (26) in each of a plurality of the images in thesequence (19).
 5. Method according to claim 1, wherein the sequence (19)of digital images is caused to be captured by a camera (2;41) uponcompletion of an initialization phase (17), the initialization phase(17) including: measuring periodic intensity fluctuations in at leastparts of images acquired by the camera (2;41) whilst camera settings arevaried, and selecting values of the camera settings at which measuredperiodic intensity fluctuations in at least a range of frequencies aredetermined to be minimal.
 6. Method according to claim 1, including:determining a correction signal corresponding to a time-varying value ofa combination of pixel values at least a number of image points in animage part other than the measurement zone (26), and decorrelating atleast one of the pixel data of the images in at least the measurementzone (26) and the time-varying average value of the combination of pixelvalues at least a number of the image points in the measurement zone(26) from the correction signal.
 7. Method according to claim 1, whereinan output representative of whether the presence of at least one peak inthe spectrum corresponding to a frequency of the periodic physiologicalphenomenon is detected is used to control a device (8,9,13) arranged toperform a function conditional on detecting a presence of at least oneliving being of at least one kind.
 8. Method according to claim 7,wherein the output is provided to a conditional access system (3,13) foruse in an authentication operation.
 9. Method according to claim 1,wherein the frequency value of at least one peak in the spectrumcorresponding to the frequency of the periodic physiological phenomenonis determined and a system (9-12) for providing perceptible output iscaused to adapt its output in dependence on the frequency signal. 10.Method according to claim 1, including providing a gating signal basedon the signal (28,30) corresponding to at least the variation in thetime-varying average value of a combination of pixel values at least anumber of the image points to an imaging system (32,34,35).
 11. Methodaccording to claim 1, including: selecting a plurality of measurementzones (26); for each measurement zone (26), obtaining a signalrepresentative of at least variations in a time-varying average value ofa combination of pixel values at least a number of the image points anddetermining the frequency value of at least one peak in a spectrum ofthe signal corresponding to a frequency of a periodic physiologicalphenomenon; and detecting how many living beings are represented in thesequence of images by comparing the frequency values.
 12. System forprocessing images of at least one living being, including: an interface(4;42) for obtaining data representative of a sequence (19) of digitalimages taken at consecutive points in time; and an image data processingsystem (3;35), configured to: select at least one measurement zone (26)comprising a plurality of image points, wherein the image dataprocessing system (3;35) is configured to select the at least onemeasurement zone (26) by analyzing information based on pixel data of aplurality of image parts in at least one of the images (19), each imagepart including at least one image point, and to select each measurementzone (26) from contiguous parts determined to have similarcharacteristics; and for each measurement zone (26), to obtain a signal(28,30) representative of at least variations in a time-varying value ofa combination of pixel values at least a number of the image points foruse in determining at least one of a presence and a frequency value ofat least one peak in a spectrum of the signal (28,30) corresponding to afrequency of a periodic physiological phenomenon.
 13. System accordingto claim 12, configured to carry out a method according to any one ofclaims 1-11.
 14. Computer programme including a set of instructionscapable, when incorporated in a machine-readable medium, of causing asystem having information processing capabilities to perform a methodaccording to claim 1.